This thread on frama-c-discuss
http://lists.gforge.inria.fr/pipermail/frama-c-discuss/2010-March/001844.html
suggests that the issue lies in the two functions/predicates declared in the axiomatic block. A workaround until this is fixed would be to declare separately isList and jLgth (putting the axioms in the latter). Note that it might imply
to write some reads clauses for isList (while they'd be inferred from the axioms in the case of only one axiomatic).